Ensure getConstantColorComponents() is tested for all effects with non-trivial implementations.

Issue description

Effects can trigger optimizations when they claim their output color components are constant via getConstantColorComponents(). (Currently, this is true only if they report that their output alpha is 1). However, this may not be tested for every effect that has a non-trivial implementation of the virtual. That is, some effects may have code that can report the alpha is 1 but aren't currently tested in a way that would turn this into a blend optimization.